Visually guided behaviour of cats in the absence of retino-geniculo-cortical pathways.

Abstract

Visually guided behaviour was studied in cats after various lesions of the central nervous system in order to obtain further information about the functional capacity and developmental plasticity of subcortical visual areas. Hemidecortications were made in all cats either within 5 days of birth (neonatal) or more than one year after birth (adult). In addition the optic chiasm was transected at another time in some cats, or the optic tract contralateral to the hemidecortication in the other cats. Visual behaviour was investigated with several tests: Locomotion among obstacles (free field). Search for openings of chambers (labyrinth). Jumping from/between boxes of different heights (jumpbox). Object vision and visual field (fish-picking cage). Visual learning (T-maze). Pupillary light reactions. Visual placing reactions. Reactions to moving visual stimuli. The hemidecortications caused different visual defects for the adults and the neonatals, which in addition showed varying prominence depending on type of test. All cats showed defects which were caused by hemianopia. Less defects were found in the free field, labyrinth and jumpbox tests for the neonatals compared to the adults. The difference is attributed to a better ability of the neonatals to utilize subcortical visual areas for visual orientation in space. The subsequent optic chiasm or optic tract sections permitted comparison between the functional capacity of crossed and uncrossed retinal fibres to subcortical structures. The uncrossed fibres were found to be unable to maintain any visually guided behaviour even for the above described neonatals. It appears likely that the better vision of the neonatally hemidecorticated cats was mediated via crossed retinal fibres to subcortical structures. It was eventually possible to demonstrate visually guided behaviour which was mediated via subcortical structures also in the adult hemidecorticated cats in the T-maze. The residual vision consisted of an ability to discriminate contrast differences. The present findings emphasize the potential functional importance of subcortical visual areas in the cat, especially after early brain damage. The differential findings from the different tests have shown the limitations of single test investigations. The presence of learnt visual behaviour in the absence of spontaneous visual behaviour for individual animals of this study advocates for greater use of spontanous behaviour for investigations of visual defects caused by brain lesions.